Re: [PATCH] powerpc/entry: Fix double accounting of user time on interrupt entry

On Wed, 2026-09-02 at 12:37 +0530, Shrikanth Hegde wrote:

Hi Shrikanth,

Hi Aboorva.

On 9/2/26 10:36 AM, Aboorva Devarajan wrote:

quoted

Since the switch to generic entry, an interrupt taken from user mode
accounts user time twice: once in arch_interrupt_enter_prepare() and
again in arch_enter_from_user_mode(), which irqentry_enter() invokes
for the same interrupt:

   arch_interrupt_enter_prepare()
       account_cpu_user_entry()
   irqentry_enter()
     arch_enter_from_user_mode()
       account_cpu_user_entry()

account_cpu_user_entry() accumulates the time spent in user mode
since the last return to user space, so the second call charges the
same interval again.

With CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y this roughly doubles the
reported user time of any workload that takes interrupts. On a
pseries LPAR, ps/top show ~200% CPU for a single-threaded CPU-bound
loop, and time(1) reports user time about twice the elapsed time.

Could you please run mpstat with 50% or less loading workload like stress-ng
and document the difference in changelog.

Sure, will add it in v2.

1. On the same pseries LPAR, taskset -c 6 stress-ng --cpu 1 --cpu-load 50:

Without the patch, the reported %usr is inflated, while with the patch
it is close to the expected 50%.

Without Patch:
06:43:09 AM  CPU    %usr**   %nice    %sys %iowait    %irq   %soft  %steal  %guest  %gnice   %idle
06:43:10 AM    6   69.74     0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00   30.26

With Patch:
06:29:33 AM  CPU    %usr**   %nice    %sys %iowait    %irq   %soft  %steal  %guest  %gnice   %idle
06:29:34 AM    6   48.51     0.00    0.00    0.00    0.99    0.00    0.00    0.00    0.00   50.50

2. Single-threaded CPU-bound workload:

taskset -c 100 python3 -c 'while True: pass' (executed for ~3 seconds)

Without patch:
TIME         ELAPSED %CPU
00:00:06       00:02  209

With patch:
TIME         ELAPSED %CPU
00:00:03       00:03  104

quoted

Remove the accounting from arch_interrupt_enter_prepare() and rely on
arch_enter_from_user_mode(), which runs for both syscalls and
interrupts. The duplicate account_stolen_time() call is removed the
same way.

Fixes: bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature")
Signed-off-by: Aboorva Devarajan <redacted>
---
Verified on a pseries LPAR (CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y),

Usual default is VIRT_CPU_ACCOUNTING_GEN, selected by NO_HZ_FULL.
Most distros usually enable NO_HZ_FULL=y.

At hindsight, I don't see the issue dependency on it. But better to be sure.

It is only observed with CONFIG_VIRT_CPU_ACCOUNTING_NATIVE.

account_cpu_user_entry() updates utime only with NATIVE accounting.
With CONFIG_VIRT_CPU_ACCOUNTING_GEN or tick-based accounting, it is
an empty stub and therefore a no-op.

So kernels using CONFIG_VIRT_CPU_ACCOUNTING_GEN or tick-based accounting
are not affected.

quoted

7.3.0-rc1, single-threaded CPU-bound loop:

Before:

   $ python3 -c 'while True: pass' &
   $ sleep 3; ps -p $! -o pid,etime,time,pcpu
       PID     ELAPSED     TIME %CPU
      4980       00:03 00:00:06  210

After:

   $ python3 -c 'while True: pass' &
   $ sleep 3; ps -p $! -o pid,etime,time,pcpu
       PID     ELAPSED     TIME %CPU
      4951       00:03 00:00:03  105

  arch/powerpc/include/asm/entry-common.h | 6 ++++--
  1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/entry-common.h b/arch/powerpc/include/asm/entry-common.h
index c5adb5006361..984294e62568 100644
--- a/arch/powerpc/include/asm/entry-common.h
+++ b/arch/powerpc/include/asm/entry-common.h

@@ -222,8 +222,6 @@ static inline void arch_interrupt_enter_prepare(struct pt_regs *regs)

  
  	if (user_mode(regs)) {
  		kuap_lock();
-		account_cpu_user_entry();
-		account_stolen_time();
  	} else {
  		kuap_save_and_lock(regs);
  		/*

@@ -426,6 +424,10 @@ static __always_inline void arch_enter_from_user_mode(struct pt_regs *regs)

  #endif
  	kuap_assert_locked();
  	booke_restore_dbcr0();
+	/*
+	 * User and stolen time is accounted here for every entry from
+	 * user mode. The interrupt prepare hooks must not account again.
+	 */
  	account_cpu_user_entry();
  	account_stolen_time();
  

base-commit: fb442a6673ff1046bf67754957d95880fdb394b5

Thanks,
Aboorva